This invention relates to voltage doubler arrangements for power supplies and particularly to automatic voltage doubler switches.
U.S. Pat. No. 4,665,323, entitled ELECTRONICALLY SWITCHABLE POWER SOURCE, issued May 12, 1987, to Russell, Salata and Stevens, and assigned to Zenith Electronics Corporation, describes a DC power source that automatically adapts to the 120 volt AC or 240 volt AC line input to produce a single range of regulated output voltage. The circuit utilizes a plurality of comparators for determining when the circuit should switch to the voltage doubler mode. It also includes a Triac that is supplied with a DC trigger voltage for switching between the two operating modes.
U.S. Pat. No. 4,783,729, entitled AUTOMATIC VOLTAGE DOUBLER SWITCH, issued 11/8/88 to the present inventor and assigned to Zenith Electronics Corporation, discloses an automatic voltage doubler switch including a Triac, the gate electrode of which is driven by a relaxation oscillator that includes a Diac switch. A capacitor is charged from a voltage that is indicative of the AC line voltage and triggers the Triac through the Diac switch to complete a conductive path for converting the power supply from a direct operating mode to a voltage doubler operating mode. A cutoff transistor senses the AC line voltage and functions to disable the relaxation oscillator when the AC line is at a particular level. A hysteresis circuit reduces the resistance in the base circuit of the line voltage sensing transistor to keep it in conduction despite further drops in line voltage. A hysteresis inhibit transistor operates on power up of the power supply to temporarily override the hysteresis circuit.
U.S. Pat. No. 4,805,083, entitled LOW COST AUTOMATIC VOLTAGE DOUBLER SWITCH, issued 2/14/89 to the present inventor and assigned to Zenith Electronics Corporation, involves a self-biasing SCR trigger circuit that senses the AC line voltage. The SCR anode and cathode are coupled across a capacitor which is connected in series with a resistor across one of the bridge rectifiers. The capacitor is part of a drive circuit for a Triac. When the DC voltage rises to a predetermined level, the SCR fires and shorts out the capacitor which renders the Triac nonconductive. The capacitor delays the drive current upon start up to permit the SCR trigger to sense the AC line voltage. The capacitor also maintains the Triac and the SCR conductive during phase reversals of the AC line. A switchable RC delay is activated in the voltage doubler mode to delay operation of the SCR trigger.
All of the above patented circuits perform their intended functions satisfactorily. The present invention primarily deals with a problem that is encountered with voltage doublers, including the above type doublers which use a Triac for switching. Due to the voltage drop across a Triac, it is not suitable for switching large currents. For a high current power supply, that is one in which the current is on the order of 10 amperes, a one volt rms. drop across the Triac will generate a 10 watt loss. Even with a very heavy duty Triac, such a loss is unacceptable for most applications. In the present invention, relay contacts are used to perform the switching without loss. A difficulty arises in that if the relay is designed to operate with a predetermined voltage, it will not operate at one-half of that voltage. On the other hand, if the relay is selected to operate at the lower voltage, steps must be taken to protect it when exposed to the full voltage.
In accordance with the invention, a transformer has a pair of similar primary windings. Relay contacts are used to place the primary windings in parallel across the input power lines for 120 volt operation and in series for 240 volt operation. The normal mode or relay contacts position is for 240 volt operation, i.e. operation in a nondoubling mode. A bridge connected rectifier coupled across the secondary winding develops normal DC voltage. A voltage doubler is also coupled across the secondary winding for developing a higher than normal DC voltage. A voltage doubler capacitor is connected across the relay operating winding or coil. The relay coil is energized by relay switching means when the voltage across the capacitor reaches a predetermined level. A voltage sensing circuit checks the normal voltage and operates to disable the relay switching means if the normal voltage rises to a level that indicates that no doubling action is needed.
The inventive arrangement may be used as an isolation type automatic voltage doubler power supply to develop a nominal voltage for both 120 and 240 volt inputs. The inventive arrangement may also be used as a relay switching circuit for switching a high current switched mode power supply (SMPS) in and out of the voltage doubler mode by means of additional relay contacts. Both uses of the invention are contemplated.